This invention relates to lignocellulose composition boards. It pertains particularly to wood composition boards of high strength and medium density, e.g. a density of 20 to 50 pounds per cubic foot.
In the building trades need exists for a composition board, specifically a wood fiber board, of medium density and increased strength which retains the other necessary and desirable fiber-board properties of dimensional stability, thickness swelling, water absorption, uniformity, and the like. If such a fiber board product were to be available commercially, it could be substituted in building materials specifications for fiber boards of substantially higher density. This would result in significant savings of raw materials, as well as in important economies in building material costs and transportation costs.
Accordingly, it is the general purpose of the present invention to provide a composition board product of medium density and greatly increased strength which is characterized also by acceptable properties of dimensional stability, thickness swelling, water resistance, and uniformity.
It is a further purpose of the present invention to provide a composition board product of low cost which can be produced in high yield from its original starting materials.
A further object of the present invention is the provision of a composition board product which does not require the inclusion together with its primary fibrous lignocellulose component of an excessively large quantity of an exotic binder of high cost.
Still another object of the present invention is the provision of a composition board product including a low cost binder which serves the ancillary functions of acting as a dispersant for the lignocellulose fiber component of the board, as a fines retention agent, and also as a board density control agent.
A still further object of the present invention is the provision of a rapid and economical process for the manufacture of composition boards having the above noted qualities of medium density and high strength.
I now have discovered that the foregoing and other objects of the present invention are achieved by the provision of a medium density composition board comprising broadly, on a dry weight basis, from 60 to 95% by weight of lignocellulose fiber and from 5 to 40% by weight of a unique cellulosic gel binder. As will be described in detail hereinafter, the cellulosic gel binder employed is exhaustively hydrated so that it is characterized in its gel condition by a TAPPI drain time of at least 350 seconds, preferably at least 900 seconds, and specifically from 900 to 2000 seconds.
The medium density board product of the invention has a density of from 20 to 50 pounds per cubic foot. However, it has strength properties which are characteristic of prior art fiber boards having a much higher density. For example, a medium density fiber board of the invention at a density of 30 pounds per cubic foot meets the current product standards of the American Hardboard Association for building siding and furniture cores for wood fiber boards having a density of 42 pounds per cubic foot. The standards met include modulus of rupture, tensile strength, thickness swelling and water absorption. The savings in raw material use, transportation costs, and product cost resulting from the substitution of the hereindescribed medium weight product in building specifications are immediately apparent.
The noted improvement in strength is achieved, furthermore, at no sacrifice of the other important properties required of fiber board products.
This desirable result is achieved as a direct consequence of the inclusion in the hereindescribed fiber board products of an exhaustively hydrated cellulosic gel binder as a major component of the board-making furnish. In the forming of the board product, the gel component not only acts as an efficient binder in a quick pressing operation to bind the particles of lignocellulose into an integral board, it also acts as a dispersant for the fibers so that a uniform board free from the presence of bunched fiber aggregates is obtained. It also shrinks during drying of the board with the result that it compacts and integrates the other board components. Being water resistant per se, it augments the water resistant characteristics of the finished board product.